It is desirable in formation of polysilicon films to provide large individual polysilicon grains to maximize resultant conductance upon impurity doping. The desire for large polysilicon grains is even greater in fabrication of thin-film-transistors (commonly referred to as "TFTs") which typically have a thickness of from about 100 Angstroms to around 2000 Angstroms.
Polysilicon can be provided atop a semiconductor wafer by direct deposition or deposition of amorphous polysilicon followed by a crystallization anneal. Another technique employs "recrystallization" of a polysilicon film, whether that film be polycrystalline as deposited or initially rendered so by a post-amorphous deposition anneal. "Recrystallization" refers to the nucleation and growth of new grains within a preexisting crystalline matrix that has been made amorphous, or to grain enlargement of preexisting grains. It is typically undesirable to directly deposit polysilicon, as such deposition inherently results in very small grains, large numbers of grain boundaries, and other undesired defects. Grain growth and size can be maximized by amorphous silicon deposition followed by crystallization anneal, or by recrystallization techniques. Larger grains result from these processes as opposed to mere direct deposition of polysilicon.
This invention comprises an improved technique for enhancing or enlarging the size of polysilicon grains in a polysilicon film.